liquid-crystal fabry -perot filters

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Wing-Kit Choi ( 蔡永傑 )PKU-NTU Joint Workshop on Silicon

Photonics,at Peking University

7/12/2013

1


Outline

1. Introduction to LC2. Introduction to PDLC/nano-PDLC3. Introduction to LCFP filters

2


Liquid Crystal (LC)A mainstream technology for today’s displays

3


Advantages of LC technologyLow voltage / low power consumptionLarge electro-optic effects/Large birefringenceNo moving part / ReliableLong lifeRobustCompactEasily scaled to large area / large number of pixels,

etc

4


What are Liquid Crystals ?Intermediate between crystalline solid and amorphous liquidUsually found in organic molecules with:

Highly anisotropic shapes, e.g. rod or disc shapeIntermolecular forces: Crystals > Liquid crystals > Liquid

5

Crystalline Solid

Amorphous Liquid

Liquid Crystal

temperature


Intermediate properties

6

Crystalline Solid Amorphous Liquid Liquid Crystal

• Highly ordered• Cannot flow• Optically anisotropic

• Highly disordered• Can flow easily • Optically isotropic

• Some degree of order• Can flow• Optically anisotropic

Fluid properties of liquids + Optical properties of solids


What so attractive about LCs ?

7

CrystalsOptical properties

Liquids Fluid properties +

Liquid CrystalsOptical anisotropy

+Molecules can be re-arranged easily by electric

fields

Large electro-optic effects are possible with only small applied voltages !


Slow response of LC (nematic)Turn-ON is Fast (can be < 1ms)

Electric field driven Turn-OFF is Slow (e.g. tens of ms)

Non-electric field drivenweak restoring force of LC molecules

A major limitation of LCs

8


How to achieve a faster LC response time ? Use of different LC PhaseModified Electrode DesignDifferent LC ModePolymer/LC e-o effects Thinner cell gapOver-Drive schemesDual frequency, etc

9


Introduction to PDLC• LC droplets dispersed in a solid polymer matrix• Most common method to produce PDLC: Polymerization-Induced Phase Separation (PIPS)

1.Mix LC with monomers 2.Cure the mixture with UV light 3.Polymerization occurs4.LC droplets form

2013/6/2610


Operation principle

11

Scattering(Dark state)

Transmission (Bright state)

neff. > np

V = 0neff. = np

V > Vth


Advantages

12

High Optical efficiency (No polarizers)Ease of Fabrication (No Alignment layers) Potentially Lower cost (No Alignment layers) Compatible with plastic substrates to form

Flexible DisplaysPolarization Independent (in normal direction)Fast Response time possible (esp. nano-PDLC)


Electrically Switchable Windows

13

Applications

Other possible applications:

Variable Optical Attenuators (VOAs) Project Displays Reflective/ Flexible Displays Tunable lens, etc


Transmission vs cell gap

14

Thicker LC cell more scattering CR , V


Nano PDLC @ High Polymer concentration

15

~ 50% polymer Max. scattering highest CR (droplet size ~ 1m )

Polymer % , Scattering , CR (droplet size )

Polymer > ~ 70%, ~ no scattering

(droplet size ~100nm)Known as nano-PDLC

50%

60%

70%


Nano PDLC Fast Response (<1ms) possible

2013/6/2616

Polymer % , Response time (droplet size )

Polymer interaction with LC stronger (more surface/volume ratio)

Polymer > ~ 70% (nano-PDLC), fast response < 1ms possible

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University 17


Fabry-Perot cavity

18

T : transmittanceR : reflectancen’ : the refractive index of the material d’ : the thickness of the etalon

Highly reflective mirror (with glass substrate)

Air ( or e.g. Liquid Crystal) Incident light

Transmitted light


Wavelength tuning

19

Tuning =~ 50nm


LCFP filtersFirst Proposed by a group at Rockwell Int. Science

Centre, US, 1981 (Gunning et al)To employ large n of LC: Highly efficient wavelength tunable filters Visible and Infrared Applications Lower Voltage Wider tuning range compared to other solid e-o

materials

20


LCFP filtersSince 1990, further improved by groups at e.g. :1) Bell Core , NJ ( Patel et al)2) NTT Optoelectronics, Japan (Hirabayashi et al) for WDM in telecommunications with Lower Loss, narrow bandwidth (<1-2nm),

wide tunable range

21


LCFP (using PA-LC)

Spectrum with Pol. And without Pol.

22

Wavelength tuning

ne

no

(Bellcore ,1990)


LCFP (Polarization Independent)

23

Split into 2 components by Calcite Spectrum vs V without Pol.

(Bellcore, 1991)


LCFP (Polarization Independent)

24

POLARIZER

POLARIZER

GLASS

LC

GLASS

Spectrum vs V without Pol.At > ~2.5V , ne and no modes merge

(Bellcore,1991)


High speed LCFP using FLC

High speed (<100s)BinaryBistable

25

(Bellcore,1993)


High speed LCFP using DHFLC

High speed (< ~ 100 s)Low Voltage May have hysteresis effect

26

(Cambridge, 1996)


High speed LCFP using Sm*A LC High speed (<10 s)High Voltage Elevated temp.Tilted alignment (complicated)

27

(Colorado, 1996)


Wing-Kit Choi ( 蔡永傑 )National Taiwan University

[email protected]: +886-2-3366-3669

28

liquid-crystal fabry -perot filters

Documents